This invention relates generally to the field of photography and, more particularly, an improved method and apparatus for controlling photoresponsiveness of exposure control systems during exposure.
Automatic light responsive exposure control systems are well-known in the photographic arts. One such automatic exposure control system employs scanning type shutter blades. Exemplary scanning shutter blades usable in such systems are disclosed generally in U.S. Pat. No. 3,942,183, issued Mar. 2, 1976, to George D. Whiteside; and U.S. Pat. No. 4,104,653, issued Aug. 1, 1978, to Bruce K. Johnson et al., all of which are presently assigned in common with the present application. As described in these patents, cooperating pairs of taking or primary, and secondary or photocell apertures are formed in the shutter blades. These pairs of apertures cooperate respectively for blocking and unblocking the passage of light through an exposure opening to a film plane, and through a photocell opening to a light sensing or photoresponsive cell used for controlling blade positioning. During the exposure cycle, the photocell apertures operate in conjunction with the photocell and a control circuit to define both the taking aperture values achieved and the exposure interval as a function of the amount of scene radiation received through the photocell apertures.
For optimizing photographic quality when using systems of the type noted, a spectral correction filter can can be employed in the photocell's optical path. Such filter correlates the spectral sensitivity curve of the photoresponsive element more closely with that of the eye. Without such a filter, it will be understood that the typical photocell would react to spectral frequencies, such as infrared (IR), so as to cause the control circuit to terminate an exposure interval earlier than desired. This is especially the case when the photocell is of the silicon type, because such a photocell tends to be red (IR) sensitive. Ordinarily then, use is made of a spectral correction filter having peak absorption in the near-infrared region and high transmission in the visual region.
While use of an infrared spectral filter serves satisfactorily, complications can arise with its usage particularly in flash exposures when reflectivities of different objects in photographic scenes exhibit widely disparate values. Partly as a result of this, it has been found advantageous to remove the infrared filter in flash exposure modes. Removal of this filter during flash firing does not, however, eliminate entirely the effect of the disparate visible reflectivity values.
It has been known to provide a fixed proportion of different spectral evaluation for improved exposure by employing a filter having select spectral filtering capabilities. While this approach can provide improvement in exposure, the manufacture of such filters is an expensive and time-consuming process requiring the maintenance of high standards for achieving consistent quality.
In copending and commonly assigned application Ser. No. 127,120, entitled "Method and Apparatus for Controlling Exposure by Selective Use of Blocking Visible Filter", filed Mar. 4, 1980, by John B. Millard which has been abandoned and replaced by continuation-in-part application having Ser. No. 204,045, filed Nov. 4, 1980, it has been proposed to use a spectral filter which, during a flash portion of the exposure interval in which the contribution of scene light is provided by the flash is important, blocks the visible frequencies, while exclusively passing the infrared frequencies. It has been found that with this arrangement, the disparate visible light reflectivity values giving rise to inaccurate evaluations of scene brightness during a flash firing are substantially reduced.
Copending and commonly assigned application Ser. No. 156,198, entitled "Method and Apparatus for Selectively Positioning Spectral Filter", filed June 3, 1980, by Bruce K. Johnson, now U.S. Pat. No. 4,315,675 discloses an exposure control system with artificial flash lighting employing a pair of photoresponsive regions having a blocking visible spectral filter for the strobe mode and a blocking infrared spectral filter in the ambient mode. In this system, the shutter mechanism is provided with a photocell aperture arrangement which, during shutter scanning, alternatively directs scene radiation to one or the other photoresponsive regions depending on the ambient light conditions.
This approach represents a significant advance in providing improved exposure under many different lighting conditions. However, even this approach has potential for less than satisfactory results in certain, relatively uncommon, scene lighting situations. For example, difficulties can arise under low ambient conditions in scenes having a relatively high amount of infrared radiation, such as produced by green plants, since the blocking visible filter is overlying the photocell during a significant amount of the exposure such that the cell exclusively receives infrared frequencies. As a result, a likelihood for underexposure exists.
This is due, in part, to the fact that with such scanning shutter systems the blocking visible filter is controlling passage of scene radiation to the photocell for a significant portion of the time the photocell circuit is integrating the scene radiation. Hence, when only infrared is being passed to the photocell, as when the blocking visible/passing infrared filter is controlling, the photocell response for terminating exposure will be accentuated due to the infrared of the green plants.
It should be mentioned that in the last-noted application, as the photocell aperture arrangement was shifted from visible to infrared evaluation, it integrated for a short time both visible frequencies and infrared frequencies. This occurred, for example, during transition of the blocking infrared filter to the blocking visible filter over the photocell. Such integration of both frequencies was considered undesirable. Also, the actual duration of this simultaneous integration in such system is negligible in terms of time and thus in terms affecting exposure. Thus, the degree of this dual integration of both visible and infrared frequencies was intended to be and was in fact minimized as much as possible.